1. Field of the Invention
The invention relates to relative wavelength calibration for excimer and molecular fluorine lasers, and particularly to a technique for using an expert computer system for compensating spectral drift of a wavelength measurement system, such as may include the use of a monitor etalon or grating spectrometer.
2. Discussion of the Related Art
Excimer lasers emitting pulsed UV-radiation are becoming increasingly important instruments in specialized material processing. The KrF-excimer laser emitting around 248 nm and the ArF-excimer laser emitting around 193 nm are currently the light sources of choice for photolithographic processing of integrated circuit devices (IC""s). The F2-laser is also being developed for such usage and emits light around 157 nm.
It is recognized herein that it is desired in the field of industrial silicon processing that each of the above laser systems become capable of emitting a narrow spectral band around a very precisely determined and finely adjustable wavelength. It is further recognized that it is desired to have techniques for reducing bandwidths by special resonator designs to less than 100 pm for semi-narrow band lasers, to less than 1 pm for narrow band lasers, and to less than 0.6 pm for very narrow band lasers. It is further recognized that it is desired to have techniques for tuning and controlling central wavelengths of emission.
In order to tune the line-narrowed output of an excimer or molecular fluorine laser system precisely to a desired wavelength, a portion of the laser beam may be directed through a wavelength measurement system (WMS), such as may include the use of an etalon spectrometer (hereinafter referred to as a monitor etalon) or a grating spectrometer. The WMS may be calibrated to an absolute wavelength reference such as by directing a portion of the laser beam to an opto-galvanic cell, absorption lamp, or by comparison with a reference laser line or lamp line. Then, with the dispersion of the WMS being known, or alternatively the free spectral range of the monitor etalon being known, an optics control module tunes the optics of the laser resonator to adjust the wavelength to a desired value.
It is recognized in the present invention that the wavelength measurement system tends to drift over time from its calibration point. That is, the measured wavelength determined from the interference pattern captured on an array detector associated with the monitor etalon, or spectrally dispersed distribution detected at the array detector due to dispersion from a grating or prism, when the beam traverses the interference or dispersive optics of the WMS fluctuates from the true wavelength of the beam as the laser operates after a calibration of the WMS to the reference line.
This drift is most notably due to the effects of the DUV or VUV laser beam striking a surface or surfaces of the interference or dispersion optics of the WMS, such as the plates of the etalon or the surface of the grating or surfaces of a dispersive prism, etc. Among these effects, the gap spacing may vary as the etalon plates are heated due to absorption of some of the incident beam. Localized heating of the grating or prism surface can cause topographic distortions. Also, moisture may come out of the etalon, prism or grating surface when it heats up. Moreover, coating layer compaction may occur on the inner surfaces of the etalon, or on surfaces of the grating or prism, if used.
There may be geometric factors causing the measured wavelengths obtained using the WMS to drift away from the true values. Among these, the orientations of optics such as the angle of tilt of the etalon may change systematically with time of laser operation relative to the surface of the array detector and/or incoming beam. The projection focus of the spectral distribution or pattern, produced by the interference of the beam at the etalon or dispersion at the grating or prism, onto the array detector may also vary with time of laser operation.
When a monitor etalon is used as a WMS, the etalon may be re-calibrated periodically, using the reference line as mentioned above, to prevent the amount of drift from approaching an intolerable amount. However, these multiple re-calibration procedures are laborious and time-consuming, and would be typically performed when the laser is undesirably taken down or offline. It is desired to have a technique for compensating optical drift of the monitor etalon that avoids laser system downtime.
Moreover, the optical drift of the monitor etalon, as well as other optical, thermal and electronic phenomena, conventionally produces increasing uncertainty and imprecision at times following the initial calibration or re-calibration procedure. It is desired to have a method of compensating optical drift of the monitor etalon or other optics of a wavelength measurement system such that the output beam of the laser system is reliably spectrally located at the desired wavelength after long or short periods of laser operation following an initial absolute wavelength calibration procedure.
It is therefore an object of the invention to provide a technique for compensating optical drift of the monitor etalon or dispersive optic of a wavelength measurement system (WMS) that avoids laser system downtime.
It is another object of the invention to provide a technique, wherein a WMS can be reliably used to precisely maintain an output beam of a laser system at a desired wavelength over short or long laser operation periods, such that even after extended periods of laser operation following an initial absolute wavelength calibration procedure, the wavelength may be measured with reliability, wherein when a monitor etalon is conventionally used, the etalon would typically drift an intolerable amount over that extended period due to beam exposure.
In accordance with the above objects, a method for compensating optical drift of a wavelength measurement system used for relative wavelength tuning of an output beam of an excimer or molecular fluorine laser system is provided including operating the laser system including generating a laser beam and directing a beam portion through the wavelength measurement system, calibrating the wavelength measurement system to an absolute reference, tuning the output beam to a target wavelength using the wavelength measurement system, detecting a measured wavelength of the output beam using the wavelength measurement system after a predetermined period of laser operation, calculating a compensated wavelength by figuring in a previously determined drift compensation value; and adjusting the wavelength of the laser beam to the target wavelength when the compensated wavelength differs from the target wavelength. The method may further include repeating detecting calculating and adjusting steps a number times after additional periods of laser operation.
A method for operating an excimer or molecular fluorine laser system at a stabilized wavelength, the laser system including a wavelength measurement system for relative wavelength tuning, including operating the laser system including generating a laser beam and directing a beam portion through the wavelength measurement system, calibrating the wavelength measurement system to an absolute reference, determining the wavelength of the laser beam, and tuning the wavelength to a target wavelength when the determined wavelength differs from the target wavelength. The wavelength determining step includes transmitting wavelength information measured by the wavelength measurement system, retrieving a drift compensation value stored as corresponding to a current laser system operating condition, and calculating the wavelength of the laser beam based on the transmitted wavelength information and the retrieved drift compensation value. The method may further include repeating the determining and tuning steps a number times after additional periods of laser operation.
A method for preparing an excimer or molecular fluorine laser system to operate at a stabilized wavelength by compensating optical drift of a wavelength measurement system used for relative wavelength tuning of an output beam of the excimer or molecular fluorine laser system is also provided including operating the laser system including generating a laser beam and directing a beam portion through the wavelength measurement system, calibrating the wavelength measurement system to an absolute reference, determining a value of the wavelength of the laser beam measured by the wavelength measurement system after a predetermined period of laser operation, comparing the value of the wavelength measured by the wavelength measurement system after the predetermined period of laser operation with an actual value of the wavelength of the laser beam and determining a drift compensation value based on a result of the comparing step. The method may further include repeating the wavelength value determining step, the comparing step and the drift compensation value determining step after additional periods of laser operation, and storing the drift compensation values versus laser operation period of the wavelength measurement system for use with a wavelength stabilization routine of the laser system.
A method for preparing an excimer or molecular fluorine laser system to operate at a stabilized wavelength by compensating optical drift of a wavelength measurement system used for relative wavelength tuning of an output beam of the excimer or molecular fluorine laser system is further provided including operating the laser system including generating a laser beam at a target wavelength by orienting a tuning optic of the laser system at a first position and directing a beam portion through the wavelength measurement system, calibrating the wavelength measurement system to an absolute reference, orienting the tuning optic to a second position such that the wavelength of the laser beam measured by the wavelength measurement system after a predetermined period of laser operation is at the target value, comparing the first position with the second position of the tuning optic, and determining a drift compensation value based on a result of the comparing step. The method may further include repeating the orienting, comparing and determining steps a number times after additional periods of laser operation and storing the drift compensation values versus laser operation period for use with a wavelength stabilization routine of the laser system.